The invention relates to an apparatus for producing a visible image of a scene by detecting nonvisible radiation from the scene. The apparatus, comprises a mirror prism which is rotatable about an axis and which includes a plurality of planar mirrors. The mirrors form an equiangular polygon in a cross-section through the prism normal to the axis. The mirrors also subtend a sequence of various tilt angles with respect to the axis. An objective lens is arranged in the apparatus to receive radiation from the scene via said planar mirrors. Radiation passes through the lens and is incident on a linear array of radiation-sensitive detectors. A linear array of light sources each source coupled to a corresponding detector in the detector array, is used to construct a visible image of the scene. Finally, viewing means is arranged to receive the light beams reflected by the planar mirrors from the light sources.
In U.S. Pat. No. 3,626,091 (Casper) a thermal imaging apparatus is disclosed comprising a linear array of infrared detectors and a corresponding linear array of light sources. Scanning of a plurality of bands of lines across the scene, and reconstruction in synchronism with the scanning are effected by a rotating mirror prism having planar mirrors set at various angles around the axis of rotation. The mirror prism has twice as many mirror faces as there are bands scanned in the scene. Scanning is carried out by a first set of mirrors while reconstruction is carried out by a second set of mirrors. This results in either bulky mirror prisms or in relatively small aperture mirrors which restrict the sensitivity of the apparatus.